1. Field of the Invention
The present invention is related to a method for quickly establishing hot test conditions representative of those expected in operational solid state LED products including packaged high-brightness light-emitting diodes (HBLEDs) or phosphor-converted HBLEDs (pc-HBLEDs) (hereinafter both called HBLEDs). The present invention is also related to a system that can rapidly provide the hot test conditions as well as high-precision measurement of optical properties of the HBLEDs.
2. Related Art
FIG. 1 illustrates an exemplary HBLED 100 including a phosphor layer 102 and a thin, e.g. a few μ thick, indium-gallium-nitride (InGaN) film 101. In typical embodiments, phosphor layer 102 is also applied to the sides of InGaN film 101. Phosphor layer 102 includes the luminescent phosphors, i.e. the microcrystals containing active visible light emitting ions. Phosphor layer 102 further includes either a binder, such as silicone, or a sintered crystal (described in further detail below). The InGaN film/phosphor layer combination is mounted on a submount 104 and then encapsulated using a lens 103 of approximately 2 mm in radius. Lens 103 is formed using silicone, which increases light extraction by closely matching the refractive index of the surface of phosphor layer 102.
HBLED 100 may be inspected at the wafer-level, either un-singulated or singulated, at varying stages of processing. When HBLED 100 is assembled into a product-level HBLED, submount 104 can be further attached to a printed circuit board as well as to a heat sink.
Generally, the photometric parameters of HBLEDs are measured during electrical probe tests. Exemplary photometric parameters include the CCT (correlated color temperature, i.e. a metric that relates the appearance of emitted light to the appearance of a theoretical heated black body that combines red, orange, yellow, white, and blue light in varying degrees to form white light in various locations along the Planckian curve), chromaticity (the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness: saturation, chroma, intensity, or excitation purity), and CRI (color rendering index, i.e. the principal metric of the CIE (International Commission on Illumination) system that uses the averaged Ri scores for eight standard test colors or similar color tests such as R96a and related tests). Probe tests are typically performed by applying a brief pulse of current to the InGaN film for a timescale typically between 10 msec and 200 msec while the optical properties of the HBLED are measured. Alternately, the electrical probe may be applied for a period of time exceeding that needed for the parametric measurement in order to attempt to bring the thermal condition of the HBLEDs to conditions more closely related to those expected in the final lighting product form.
Unfortunately, the use of electrical probes does not bring the HBLEDs to conditions anywhere near those expected in the final lighting product. The primary difficulty is due to the disparate thermal qualities of the materials used in the construction of the HBLEDs. These materials may include InGaN films, silicon or copper submounts, sapphire or SiC substrates, quartz materials for lenses or optical windows, and silicone encapsulants used to apply a slurry of microcrystalline paste, in which the microcrystals contain luminescing materials such as europium and cerium trivalent ions. Of particular note is an organic silicone phosphor carrier paste, which has a thermal conductivity at least two orders of magnitude and in most cases three orders of magnitude lower than the other enumerated materials, thereby resulting in a physical thermal time constant or response time of roughly three orders of magnitude longer than the other materials.
For example, InGaN film 101 has an operating temperature of roughly 60° C. above room temperature (85° C.), whereas phosphor layer 102, when including silicone, may reach an operating temperature of roughly 200° C. or more in some regions of some products. Note that the thermal time constant, i.e. the time to reach thermal equilibrium, for InGaN film 101 is roughly 10 msec, whereas for phosphor layer 102 can be from one to two seconds or longer. Not only are the thermal time constants different for the varying regions of HBLED 100 but, due to different dimensions and volumes, so are the heat capacities of the various regions. For both reasons, phosphor layer 102 is slower to heat than InGaN film 101.
Obtaining measurements for optical parameters with the materials at temperatures substantially different from anticipated operating temperatures of the final packaged product in a full luminiere yields incorrect results because the emitting wavelength and efficiency (intensity) of InGaN film 101 is moderately temperature dependent. More importantly, the absorption and emission spectra of the active phosphor ions in phosphor layer 102 as well as the quantum yield for the stokes-shifted emission radiation are also temperature dependent. It is therefore important that HBLED photometric properties (those related to the response of the human eye) be measured and reported at conditions as close to those anticipated in the final product as is possible.
Note that using electrical current from a probe to produce InGaN emission will bring InGaN film 101 to operating temperature within roughly 10 msec. The emitted blue radiation from the InGaN is absorbed by the active ions in the phosphors of phosphor layer 102, which in turn generates red or green or yellow (typically using (Eu+2) or (Ce+3) radiation from various host materials) as well as waste heat within microcrystals of the phosphor binder due to the Stokes shift of phosphor absorption and emission wavelengths and due to the temperature dependant non-radiational decay of each phosphor. However, the slow thermal response of the surrounding silicone in phosphor layer 102 requires that a full one to two seconds of excitation be maintained before the active phosphor ions can fully bring the surrounding silicone to the expected equilibrium operating temperature of nearly 200° C. in the surface adjacent to lens 103 (i.e. furthest from the InGaN film).
However, applying electrical excitation for one or two seconds to InGaN film 101 is not commercially attractive. Specifically, a high throughput tool needs to complete its measurement on the InGaN film within roughly 50 msec to process roughly seven 4-inch wafers (each containing roughly 10,000 die) per hour. Additionally, applying current for this duration heats InGaN film 101 well above its expected product operating temperature because the heat capacity of the film-submount is not sufficient to absorb the applied energy. To address this heating issue, HBLED 100 can be attached onto, for example, an extruded aluminum heat sink which adds heat capacity and convectively cools the overall structure in the product-level HBLED. However, this means that each HBLED 100 is singulated and then essentially packaged to near product form before it is tested. Thus, instead the film-submount at the wafer level is only attached to a film-frame carrier and therefore may only be exposed to current for 10-20 msec prior to exceeding its expected operating temperature. Thus, the application of heating energy to HBLED 100 through electrical current is insufficient to produce the expected operating conditions in the product-level HBLED. Thus, there is no accurate, commercially-viable hot testing of HBLEDs on the market today.
To address this shortcoming, the LED industry has used alternative tests. For example, in one test mode, the InGaN film is fabricated at the wafer level, then mounted onto a common roughly 1.6 mm thick alumina submount also at the wafer level. This alumina submount with widely spaced arrays thereon (mounted arrays) is next covered with a film of phosphors distributed in a silicone resin binder. Then, these mounted arrays are placed into an oven to a temperature of approximately 85° C. The mounted arrays are then powered electronically for roughly 10 milliseconds. Note that within these 10 msecs, the InGaN film and the phosphor region remain at approximately 85° C. At this point, the emission spectrum and average CIE coordinates for the entire alumina submount are recorded with all regions being nominally 85° C.
Unfortunately, with these temperature conditions for the measurement, color coordinate measurement accuracy is sacrificed. Specifically, it is well known that the temperature of the phosphor layer varies in the final product from roughly 85° C. near the InGaN film to temperatures as much as approximately 200° C. in the regions in contact with the thickest portion of the lens. The higher temperatures in the hottest regions of the phosphor layer give rise to increased non-radiative decay of the active ions and reduced conversion of blue pump radiation to longer wavelength phosphorescence, thus significantly shifting the final color coordinates of the HBLED (see FIG. 2). The exact amount of this shift is affected by the phosphor thickness, phosphor doping level, phosphor type, and additional factors. Many MacAdam ellipses of shift occur in changing the HBLED operating conditions from room temperature to 85° C. The shifts accelerate at higher temperatures.
As known by those skilled in the art, a MacAdam ellipse is an elliptical region centered on a target color on a chromaticity diagram. Each ellipse defines thresholds at which color difference becomes perceivable to the human eye. The sizes of the MacAdam ellipses are in steps, wherein any point on the boundary of a 1-step MacAdam ellipse represents one standard deviation of human perception of color mismatch between two test samples. Colors on the order of 2-step MacAdam ellipses of matching are generally considered desirable for high quality lighting applications. Color differences of a larger number of MacAdam ellipses are considered to be undesirable for high quality lighting in achromatic side-by-side lighting applications. Therefore, product bins of 5 MacAdam ellipses, and some insist as few as 3 ellipses, in size are not commercially attractive.
One relatively accurate technique for binning and color control includes using plates of phosphors, which are carefully hand-selected and combined with different luminescent emitters to achieve two-step MacAdam ellipse bins. Adjustable screws of phosphor converter can also be inserted into each device and tuned by moving their position at the top of the light producing chamber so that the bulb color coordinates are hand tuned. These hand-crafted LEDs are built one at a time in a fairly high-cost manufacturing process involving some trial and error. Currently, achieving product bins of 3-4 MacAdam ellipses in size, even using this intensive manufacturing technique, is difficult.
As shown above, current manufacturing processes are incapable of delivering fast, accurate, commercially-viable testing and binning for lighting applications. Therefore, an improved method is needed to produce operating conditions similar to those of the product-level HBLED, thereby allowing measurements relevant to customers.